There has been a surge of interest in adapting LTE radio access networks to cater for different deployment scenarios. A new innovative trend is to provide “connectivity from the sky.” To provide such connectivity, platforms being considered include drones, balloons, aircrafts, airships, and/or satellites, among others. LTE standards have been designed primarily for a terrestrial network and have not been optimized to provide “connectivity from the sky.” In particular, compared to traditional terrestrial LTE infrastructure that may generally be stationary, these new platforms in the sky are usually non-stationary and may move at high speeds.
In an LTE radio access network, a wireless device or user equipment (UE) typically follows the following access procedures.                1) Cell search: Search and acquire synchronization to a cell in the network.        2) System information reading: Receive and decode system information necessary for proper operation in the network.        3) Random access: Once the system information has been correctly decoded, UE can perform random access to access the network.        
In an existing LTE random access design, random access may serve multiple purposes such as initial access when establishing a radio link, transmitting scheduling requests, etc. Among others, one objective of random access may be to achieve uplink synchronization, which may be important for maintaining uplink orthogonality in an LTE network. To preserve orthogonality among different UEs in an OFDMA or SC-FDMA system, the time of arrival (ToA) of each UE's signal may need to be within the cyclic prefix (CP) of the OFDMA or SC-FDMA signal at the base station.
LTE random access can be either contention-based or contention-free. A contention-based random access procedure may generally include four operations. Brief reference is made to FIG. 8, which is a signal flow diagram illustrating a contention-based random-access procedure. Note that the first operation involves physical-layer processing specifically designed for random access, while the random-access response 810, the scheduled transmission 815 and the contention resolution 820 may follow the same physical-layer processing used in uplink and downlink data transmission. For contention-free random access, the UE uses reserved preambles assigned by the base station. In this case, contention resolution is not needed, and thus only operations 805 and 810 may be required.
Still referring to FIG. 8, in terrestrial LTE networks, the eNB estimates a ToA from a received Msg1 transmitted by UE in operation 805. Based on the estimate, eNB feeds back the acquired uplink timing in Msg2 to command the UE to perform timing advance in operation 810. This timing advance mechanism may help ensure that the subsequent uplink signals from UEs located in different positions in the cell can arrive at the base station within the CP range.
According to LTE standards, random access preambles may be sent in physical random-access channel (PRACH). The PRACH subcarrier spacing may be 1.25 kHz and the preambles may be Zadoff-Chu (ZC) sequences of length 839. A fixed number of preambles may be available in each LTE cell. For example, 64 preambles may be available in each LTE cell. Several preamble formats of different durations of the sequence and cyclic prefix (CP) may be defined to be used for cells of different sizes. The format configured in a cell may be broadcast in the System Information.
ZC sequences have also been used as reference signals for other purposes in LTE networks as well. For example, ZC sequences may be used in the 3 primary synchronization signals (PSS) in LTE networks. There may be 3 length-63 ZC sequences for the PSS and the length-63 ZC sequences may be extended with 5 zeros at the edges and mapped to the center 73 subcarriers in LTE. Together with secondary synchronization signals (SSS), PSS may enable the UE to acquire frequency and symbol synchronization to a cell, frame timing of the cell, and physical-layer cell identity of the cell.
ZC sequences may also be candidates for reference signals in 5G new radio (NR), such as downlink synchronization signals and uplink random access preambles.
It is known that the ambiguity function of ZC sequences does not have the “thumbtack-like” property, which features a single central peak in the Delay-Doppler plane. In particular, there are several peaks in the ambiguity function of ZC sequences in the Delay-Doppler plane, leading to many timing and Doppler ambiguities when ZC sequences are used to acquire timing and frequency synchronization.
Using LTE PRACH as a specific example, LTE PRACH subcarrier spacing is 1.25 kHz and may not handle a Doppler shift (plus residual carrier frequency offsets) larger than 1.25 kHz. Due to the nature of ZC sequences, both delay and frequency shift cause cyclic shift in the observation window of received ZC sequences at the eNB. As a result, two issues may arise. First, it may be difficult if not impossible to separate the two effects (delay and frequency shifts) by observing the composite cyclic shift. Separating the delay and frequency shift may be advantageous, however, to provide estimates thereof. Second, the composite cyclic shift may make sequence A be interpreted as sequence B, leading to misdetection. Although LTE PRACH cyclic shift set has been designed to avoid this situation, the capacity to avoid misdetection may be limited to frequency shifts that are less than 1.25 kHz, which is not sufficient for LTE deployments in high Doppler scenarios. Such scenarios may include a moving base station in the air, such as a satellite or other airborne device.
Approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the Background section are not prior art to the inventive embodiments disclosed in this application and are not admitted to be prior art by inclusion in the Background section. Therefore, any description contained in the Background section may be moved to the Detailed Description section.